1. Electric
Current
And
Ohm’s law

2. Electric Current Current = flow of charges unit: Amperes (A) symbol: I
(The rate at which charge flows by
a given cross section)
I = 𝑄 𝑡

3. Electric Current Current = flow of charges unit: Amperes (A) symbol: I
(The rate at which charge flows by
a given cross section)
I = 𝑄 𝑡
To have an electric current, you need two things:
A closed circuit / path for the charges
A power supply maintain the potential difference

4. Electric Current Current = flow of charges unit: Amperes (A) symbol: I
(The rate at which charge flows by
a given cross section)
I = 𝑄 𝑡
To have an electric current, you need two things:
A closed circuit / path for the charges
A power supply maintain the potential difference
Remember: Current will flow from high potential to low potential, but charges need to be pushed (by an energy source) back from low potential to high potential.

5. Closed vs. Open Circuits
No, the circuit needs to go from + to -
No, the switch is open, so the path is not complete
Maybe! If one battery has higher V than the other
Yes, charge will flow, BUT the light will not light

6. Direction of Current
Current is defined as the direction positive charges would flow
From high potential to low potential
From + side of battery to – side of battery
Fun Fact
Benjamin Franklin defined current in this manner long before we knew much about charges.
Now, we know that positive charges stay put and negative charges flow. So, electrons actually flow opposite current.

7. An important note about current:
Calculating Current
Current is the rate of charge flow per unit time
I = 𝑄 𝑡
So, if you have 6 C of charges passing through a section of wire every 2 seconds, then your current is:
I = 6C / 2 s = 3 A
An important note about current:
Although potential decreases across a circuit, current is the same everywhere in a circuit!

8. Circuit Analogy – CFU

9. Circuit Analogy - CFU E F B C A

10. Electrical resistance (symbol R)
Electrons flow due to potential difference. If the potential difference is removed, then
As electrons move, they bump into other atoms, this slows them down and impedes their motion. .
path
atoms
(actually positive ions)
free electron
Resistance (R) is a measure of the degree to which an object impedes the flow of current.
Resistance is measured in Ohms ()

11. OHM’S LAW - Current, Voltage and Resistance
Current = the potential difference applied across a circuit divided by the total resistance of the circuit.
R - resistance
I – current V – potential difference across R

12. Examples
If a 3 volt flashlight bulb has a resistance of 9 ohms, how much current will it draw?
I = V / R = 3 V / 9  = 1/3 Amps
If a light bulb draws 2 A of current when connected to a 120 volt circuit, what is the resistance of the light bulb?
R = V / I = 120 V / 2 A = 60 

13. Check for Understanding
Which of the following would cause the current in a circuit to decrease the most?
Increased voltage and increased resistance
Increased voltage and decreased resistance
Decreased voltage and decreased resistance
Decreased voltage and increased resistance

14. Check for Understanding
If the resistance of a circuit were tripled, then the current through the circuit would be ____.
one-third as much
three times as much
3. unchanged
nonsense! There would be no way to make such a prediction.

15. Effects of electric current on the BODY- electric shock
Just for Fun …
Effects of electric current on the BODY- electric shock
Current (A)
Effect
0.001
can be felt
0.005
painful
0.010
involuntary muscle contractions (spasms)
0.015
loss of muscle control
0.070
if through the heart, serious disruption; probably fatal if current lasts for more than 1 second

16. human body resistance varies:
100 ohms if soaked with salt water;
moist skin ohms;
normal dry skin – ohms,
extra dry skin – ohms.
What would be the current in your body if you touch the terminals of a 12-V battery with dry hands?
I = V/R = 12 V/ W = A quite harmless
But if your hands are moist and you touch 12 V battery, how much current would you draw?
I = V/R = 12 V/1000 W = A
a dangerous amount of current.